Intermolecular and Interfacial Origins of Layering in a Near-Isotropic Organic Glass Former
Physical vapor deposition can produce organic glasses with enhanced stability and pronounced structural anisotropy (layering) even when the constituent molecules are nearly spherical. However, the microscopic origin of the layering in spherical molecular glass formers remains unclear. We investigated vapor-deposited films of 9-(3,5-di(naphthalen-1-yl)phenyl)anthracene (α,α-A) by grazing-incidence wide-angle X-ray scattering, atomistic molecular dynamics, and coarse-grained simulations. Experimentally, the layering signal varies nonmonotonically with deposition temperature and is strongest near 0.8T_g. Simulated scattering calculations reproduce the principal experimental features and show that the layering peak does not originate from shifts in single molecule degrees of freedom. In fact, the simulated deposited and bulk glasses show largely the same single-molecule conformations. Interestingly, a one-site coarse-grained liquid with isotropic interactions develops a comparable interfacial layering signature. These results indicate that layering in α,α-A arises primarily from collective density correlations generated at the free surface during deposition rather than from conformational bias or orientational ordering. The findings connect structural anisotropy in vapor-deposited molecular glasses to the generic interfacial organization of simple liquids.